1. Field of the Invention
This invention relates to a contact material for a vacuum interrupter and a method for producing the same, and more particularly to a contact material for a vacuum interrupter which can improve the high current-interrupting characteristic, the current chopping characteristic and the high current-carrying characteristic of a vacuum interrupter and a method for producing the contact material for a vacuum interrupter.
2. Description of the Related Art
The contacts of a vacuum interrupter which causes the breaking of a current in a high vacuum, using the arc diffusion in a vacuum, are composed of two contacts which face each other, one fixed and the other moving. When breaking the current of an inductive circuit, such as an electric motor load, using this vacuum interrupter, there is sometimes a risk of damaging the load device through the generation of an excessive abnormal surge voltage.
Causes of generation of this abnormal surge voltage are, for instance, the chopping phenomenon which generates during the breaking of a small current in a vacuum (the phenomenon which forcibly breaks the current without waiting for the natural zero point of an AC current waveform) or the high-frequency arc-extinguishing phenomenon. A value Vs of the abnormal surge voltage due to the chopping phenomenon is indicated by Zo.multidot.Ic, where Zo is a surge impedance of a circuit, and Ic is a current chopping value. Therefore, in order to decrease abnormal surge voltage Vs, current chopping value Ic must be reduced.
As contacts which have low current chopping characteristics, there are, mainly, Cu-Bi alloy contacts which are produced by the melting method and Ag-WC alloy contacts which are produced by the sintered infiltration method.
The commonly-known Ag-WC alloy contacts exhibit superior low chopping current characteristics in, such points as:
(1) the presence of WC helps the electron emission; PA1 (2) the evaporation of the contact material is accelerated based on heating the electrode surface due to the collision of electric field emitted electrons; and PA1 (3) the carbide of the contact material is decomposed by the arc and connects the arc by forming a charged body. Vacuum switches which use these alloy contacts have been developed and put into actual use.
Also, Ag-Cu-WC alloys have been proposed (Japanese Patent Publication Showa 63-59212) by compounding Cu in these alloys, in which the ratio of Ag and Cu is about 7:3. Since the ratio of Ag and Cu is selected in these alloys which does not exist in prior art, these alloy contacts exhibit stable current chopping characteristics.
Furthermore, it is suggested in Japanese Patent Publication Heisei 5-61338 that making the grain size of an arc-proof material (for instance the grain size of WC) 0.2-1 .mu.m is effective in improving the low chopping current characteristic.
On the other hand, with Cu-Bi alloy contacts, the current chopping characteristic is improved by the selective vaporization of Bi. Out of these alloys, an alloy (Japanese Patent Publication Showa 35-14974) in which Bi is included by 10 weight % (hereafter, written as "wt %") exhibits a low current characteristic, since it has a suitable vapor pressure. Also, in an alloy in which Bi is included by 0.5 wt % (Japanese Patent Publication Showa 41-12131), Bi exists with segregation at the crystal grain boundaries. As a result, by weakening the alloy itself, this alloy achieves a low welding separation force, and therefore has a superior large current-interrupting property.
However, in its original role, a vacuum circuit breaker must perform the large current-interrupting. For this large current-interrupting, it is important to reduce the thermal input per unit surface area of the contact material by igniting the arc on the whole surface of the contact material. As a means for this, there is an axial magnetic field composition in which a magnetic field is generated parallel to the inter-electrode electric field in the electrode parts on which the contact materials are mounted. According to Japanese Patent Publication Showa 54-22813, by suitably generating a magnetic field in such a direction, it is possible to uniformly distribute the arc plasma on the contact surfaces. As a result, it is possible to increase the large current-interrupting performance. Also, concerning the contact material itself, according to Japanese Patent Disclosure Heisei 4-206121, the mobility of arc cathode points can be improved by making the WC-Co inter-granular distance in Ag-Cu-WC-Co alloy contact materials about 0.3-3 .mu.m thereby to improve the large current-interrupting characteristic. Moreover, it is indicated that by increasing the content of Iron Group auxiliary components, such as Co, the current-interrupting performance can be increased.
A low surge characteristic is required in vacuum circuit breakers and, as a result a low chopping current characteristic is conventionally required, as described above. However, recently the application of vacuum interrupters to induction type circuits, such as large capacity electric motors, is increasing. Furthermore, high surge impedance loads have also appeared. Therefore, for a vacuum interrupter, it is desirable to have an even more stable low chopping characteristic, and it must also be provided with a large current-interrupting characteristic.
However, in the case of an alloy in which 10 wt % of Bi and Cu are included (Japanese Patent Publication Showa 35-14974), with increasing the number of switchings, the supply of metal vapor is decreased in the electrode space, as a result, deterioration of the low chopping current characteristic occurs. Deterioration of the withstand-voltage characteristic, which depends on the quantity of high vapor pressure elements, is also pointed out.
In the case of an alloy in which 0.5 wt % of Bi and Cu are included (Japanese Patent Publication Showa 41-12131), the low chopping current characteristic is insufficient. It is thus impossible to have a stable low chopping current characteristic only by the selective vaporization of high vapor pressure components. In the case of contact materials which include Ag as a conductive component, such as Ag-WC-Co alloy, although they exhibit comparatively superior chopping characteristic, sufficient current-interrupting performance cannot be obtained due to the vapor pressure being excessive.
Also, in contact materials which have a conductive component with Ag as the main component, such as Ag-Cu-WC alloy in which the weight ratio of Ag and Cu is roughly 7:3 (Japanese Patent Publication Showa 63-59212) or alloys out of these alloys in which the grain size of an arc-proof component, such as WC, is 0.2-1 .mu.m (Japanese Patent Publication Heisei 5-61338) although they exhibit comparatively superior chopping characteristic and current-interrupting characteristic, the prices of these contacts are high because these contacts include expensive Ag as a conductive component. Moreover, in the case of designing improvement of the current-interrupting performance by increasing the Co content of these contact materials, the low chopping current characteristic is impaired due to the increase of the Co content.
On the other hand, in the case of using inexpensive Cu as the conductive component, the current-interrupting performance becomes comparatively good, but good chopping current characteristics cannot be obtained unless the arc-proof component is increased. For instance, in the case of Cu-WC-Co alloy, by adding Co during sintering of the WC skeleton, the porosity of the WC skeleton is reduced and the amount of Cu which can infiltrate the void is suppressed.
However, the sintering activators, such as Co, Fe and Ni for carbides, such as WC, reduce the conductivity of Cu. Therefore, the current-carrying characteristic is greatly impaired.